Applied Thermal Engineering 29 (2009) 2680–2688

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Applied Thermal Engineering

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Combustion characteristics of methanol–air and methanol–air–diluentpremixed mixtures at elevated temperatures and pressures

Zhiyuan Zhang, Zuohua Huang *, Xiangang Wang, Jianjun Zheng, Haiyan Miao, Xibin WangState Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, People’s Republic of China

a r t i c l e i n f o a b s t r a c t

Article history:Received 21 July 2008Accepted 22 December 2008Available online 1 January 2009

Keywords:MethanolPremixed mixturesDiluentCombustion characteristics

1359-4311/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.applthermaleng.2008.12.028

* Corresponding author. Tel.: +86 29 82668726; faxE-mail address: zhhuang@mail.xjtu.edu.cn (Z. Hua

Combustion characteristics of the methanol–air premixed mixtures were studied in a constant volumebomb at different equivalence ratios, initial pressures and temperatures, and dilution ratios. The resultsshow that the combustion pressure, the mass burning rate and the burned gas temperature get the max-imum value at the equivalence ratio of 1.1 while the flame development duration and the combustionduration get the minimum value at the equivalence ratio of 1.1. The flame development duration, thecombustion duration and the peak combustion pressure decrease with the increase of the initial temper-ature, while the maximum burned gas temperature increases with the increase of the initial temperature.The peak combustion pressure and temperature increase with the increase of the initial pressure. Theflame development duration and combustion duration increase with the increase of the dilution ratio,while the peak combustion pressure and temperature decrease with the increase of the dilution ratio.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Alternative fuels for both the spark ignition (SI) and the com-pression ignition (CI) engines become an important research direc-tion owing to an increased environmental protection concern, andthe need to reduce the dependency on petroleum oil promotes theresearch work [1]. Various types of alternative fuels, includinghydrogen, compressed natural gas (CNG), liquid petroleum gas(LPG), alcohols and dimethyl ether (DME), etc, are used to realizethe high-efficient low-emissions combustion engines [2–8]. Meth-anol is the simplest alcohol containing one carbon atom per mole-cule. Methanol can be produced from the coal with relatively lowprice. It has high octane number which makes it an ideal fuel forspark-ignition engines. Methanol has high oxygen content and thisis favorable to the reduction of CO and HC emissions, and methanolhas been using as an alternative fuel in automotive enginesworldwide.

In the utilization of methanol fuel, a lot of work was concen-trated on the combustion and emission characteristics of the en-gines fueled with gasoline–methanol or pure methanol [7–8].There are also some studies in the application of diesel/methanolblend and animal fats/methanol blend in compression ignition en-gine in the literature. [9–10]. Moreover, some previous researcheson the combustion applications and premixed combustion ofmethanol–air mixtures and gasoline–methanol–air mixtures were

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also reported [11–17], including the reduced methanol kineticmechanisms, the flame speed and the flame behavior.

Nevertheless, there are still many aspects for the premixedflames of the methanol–air mixtures needed to be clarified, suchas analysis of heat release (or mass burning rate) and burningvelocities (flame development duration and combustion duration).The similar parameters in the spark-ignition engines are importantfor a quantitative analysis of engine behavior. Several key parame-ters in engine combustion analysis are the maximum cylinder gaspressure, the maximum mean gas temperature, the heat releaserate, the flame development duration and the combustion durationetc. These parameters are depended on the equivalence ratio, thedilution ratio, the initial temperature and pressure. It is importantto provide some detailed engine-related information for engineMAP optimization fueled with methanol under various equivalenceratios, gas dilution ratios, initial temperatures and pressures etc.However, it is hard to directly know the influence from individualparameters and the initial condition on combustion in engine oper-ation as multi-parameter jointed influence [18]. This individualparameter influence can be indentified by using a constant volumevessel as mixture composition and initial temperature and pres-sure can be accurately controlled. It is easy to establish quantita-tively the individual effect of these initial condition andcomposition parameters on the combustion characteristics. More-over, only small quantities of combustible mixture are required forthe constant volume vessel, so this method is economic. The dataand combustion characteristics obtained from the constant volumevessel experiments not only can be used as reference to engineexperiments but also help the understanding of the influence of

Nomenclature

A wall surface area (m2)Af spherical flame front area (m2)C convective heat transfer constanth enthalpy (J)K radiant heat transfer constantLc characteristic length (m)m mass (kg)p pressure (Pa)Qb the amount heat transfer from burned gas to unburned

gas (J)Qu the amount of heat transfer to the wall (J)qr radiant heat transfer flux (J m�2 s�1);Qr the amount of heat release by fuel combustion (J);R gas constantRe Reynolds numbert time (s)T temperature (K)u internal energy (J/kg)

V volume (m3)v velocity (m/s)l dynamic viscosity (Pa s)r the Boltzmann constant, r = 5.67 � 1011 kW m�2 K�4

q density (kg m�3)k gas conductive coefficient(kW m�2 K�1)u equivalence ratiour gas dilution ratio

Subscriptsair airb burnt gasdiluent diluentfuel fuelu unburnt gasw wallst stoichiometric condition

Z. Zhang et al. / Applied Thermal Engineering 29 (2009) 2680–2688 2681

individual parameter on premixed methanol–air combustion. Theinformation can also guide the engine experiments. The combus-tion in spark-ignition engines is carried out on the elevated tem-peratures and pressures ambient environment. Thus, thecombustion characteristics at the elevated temperatures and pres-sures in the combustion vessel can provide much more data andreference information for engine combustion.

Based on the above considerations, the objective of the study isto clarify the combustion characteristics of the methanol–air pre-mixed mixtures at elevated initial pressures and temperaturesbased on the analysis of the combustion parameters. The influ-ences of equivalence ratio, gas dilution ratio, initial temperatureand pressure on the normalized mass burning rate, the flamedevelopment duration, the combustion duration and the maxi-mum burned gas temperature are analyzed.

2. Experimental setup and procedures

Fig. 1 shows the experimental setup in the study. The experi-mental system is composed of a constant volume vessel, the heat-ing system, the ignition system, the data acquisition system andthe high-speed schlieren photography system. The combustionchamber used in this experiment is a hollow cylinder type with in-ner diameter of 180 mm and volume of 5.5 l as shown in Fig. 2. Thecentrically located electrodes are used to ignite the combustiblemixture. The pressure transmitter, thermocouple, pressure trans-ducer, liquid fuel injection valve, the inlet and outlet valve aremounted on the chamber body. Two pieces of quartz windowswith the diameter of 80 mm are mounted on the two sides of thevessel, which are transparent to make the inside observable andoptical accessible. The partial pressures of each component are reg-ulated by the mercury manometer when the initial pressure of themixtures in the vessel is less than or equal to 0.1 MPa. The partialpressures are regulated by the pressure transmitter at initial pres-sures greater than 0.1 MPa. The entire vessel was heated by a2400 W heating-tape wrapped outside the chamber body. Thermo-couple is employed to measure the initial temperature of mixturesin the vessel with the measurement accuracy of 1 K. The goal initialtemperature of mixture is adjusted by a thermo-regulator. Whenthe mixture is heated to the designated initial temperature, thepower is turned off to stop the heating. The required liquid fuelis injected into the chamber corresponding to the given initial tem-perature, initial pressure, equivalence ratio and dilution ratio. Dry

air and diluent are supplied into the chamber through the inlet/outlet valve. five to 10 min are awaited to ensure the mixturesmotionless and homogeneously at the ignition timing.

The initial pressure of methanol–air–diluent mixture is,

pu ¼ pair þ pfuel þ pdiluent ð1Þ

where pair, pfuel and pdiluent are the partial pressures for air, vaporizedfuel and diluent, respectively.

The gas dilution ratio is defined as the ratio of the dilution gaspartial pressure to that of the total mixture,

/r ¼pdiluent

pdiluent þ pair þ pfuelð2Þ

Mixture equivalence ratio u is defined as,

/ ¼ ðmair=mfuelÞst

ðmair=mfuelÞð3Þ

where mair and mfuel are the amount of air and fuel, respectively, andsubscript ‘‘st” represents the stoichiometric condition for the com-plete combustion. mair and mfuel are calculated by,

mair ¼pair � VRair � Tu

ð4Þ

mfuel ¼pfuel � VRfuel � Tu

ð5Þ

where Rair and Rfuel are the gas constants of air and fuel, respec-tively. V is the volume of bomb, and Tu is the initial temperatureof mixture.

When initial pressure, Pu, initial temperature, Tu, dilution ratio,ur and equivalence ratio, u are determined. Pair, Pdiluent and mfuel canbe calculated from Formulae ((1)–(5)).

2.1. Calculation model

Calculation model is based on the hypothesis of sphericalflame. The spherical flame front divides the combustion chamberinto the burned zone and the unburned zone as shown in Fig. 3.The symbol P, T, V, and m represent the pressure, the tempera-ture, the volume, and the mass of the chamber gases, respec-tively, and Qr is the amount of heat release by fuelcombustion. The subscripts u and b represent the unburnedand burned states, respectively. Following assumptions are givenin the model,

Fig. 1. Schematic representation of the experimental setup.

Fig. 2. Schematic diagram of constant volume vessel.

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(1) gases are regarded as the ideal gases,(2) complete combustion finishes very rapidly when the

unburned charge enters the burned zone,

Fig. 3. Schematic diagram of two-zone model.

(3) pressure reaches its equilibrium value instantaneously, andthere is no difference between the burned and unburnedzones,

(4) no gas leakage is occurring, and the gas temperature reachesits respective temperature in the burned and unburnedzones,

(5) the unburned gases are regarded as the mixture of methanol,air and nitrogen,

(6) gas properties of unburned and burned gases are the mixtureproperties calculated by the fraction of the constituted gases.

According to the mass conservation and energy conservation,the following equations can be established

dmu

dt¼ � dmb

dtð6Þ

dðmuuuÞdt

¼ �PdVu

dtþ dmu

dthu þ

dQu

dtð7Þ

dðmbubÞdt

¼ �PdVb

dtþ dmb

dthb þ

dQb

dtþ dQr

dtð8Þ

Because the same pressure is considered in both the burned and un-burned zones, from the ideal gas assumption, we have,

P ¼ mbRbTb

Vb¼ muRuTu

Vuð9Þ

Z. Zhang et al. / Applied Thermal Engineering 29 (2009) 2680–2688 2683

From Eqs. ((6)–(8), (9)), the following equations can be derived,

dTu

dt¼

TuP

dPdt þ

_QumuRu

1Ru

@uu@Tuþ 1

ð10Þ

dmb

dt¼ _Q b þ _Q r þ muRu

dTu

dt� PdV

� �1Rb

@ub

@Tbþ 1

� ��

�VdPdt

1Rb

@ub

@Tbþmb

V@ub

@Tb

� �þ Vu

V

�

� ub � uuð Þ þ Ru

RbTu � Tb

� �@ub

@Tb

� ��1

ð11Þ

dVb

dt¼ Vu �

1mu

dmu

dt� 1

Tu

dTu

dtþ 1

pdPdt

� �þ dV

dtð12Þ

2.2. Heat transfer calculation

The model takes into account both convective heat transfer andradiation. The coefficient of convective heat transfer is derived fromthe plate-plate convective heat transfer correlation as follows:

a ¼ CkLc

Re ð13Þ

in which k is the gas conductive coefficient in kW m�2 K�1, Lc is thecharacteristic length, and Re is the Reynolds number, whereRe = qvLc/l.

Fig. 4. Combustion pressure and normalized mass burning rate versus the time atdifferent equivalence ratios.

The radiant heat transfer flux, qr, is calculated by

qr ¼ KrðT4 � T4i Þ ð14Þ

in which r is the Boltzmann constant, r = 5.67 � 1011 kW m�2 K�4 .Thus, the transient heat transfer to the wall, dQu/dt, and the

transient heat transfer from burned gas to unburned gas, dQb/dt,are determined by

dQu

dt¼ A½aðTu � TwÞ þ KrðT4 � T4

wÞ� ð15Þ

dQb

dt¼ Af ½aðTb � TuÞ þ KrðT4

b � T4uÞ� ð16Þ

This is the typical Annand’s heat transfer formula. Here, the con-stant K uses the value of 1.5; A is the wall surface area; and Af isthe spherical flame front area, which can be calculated byAf ¼ ð4pÞ

13ð3VbÞ

23. Tu and Tb are the gas temperature for unburned

and burned zones, while Tw is the wall temperature.In the Eqs. ((10)–(12)), dp/dt is obtained from the pressure data

in the experiments and dV/dt takes the value zero for the constantvolume bomb. The initial unburned gas temperature Tb uses theadiabatic flame temperature Tad, using the fourth-order Runge–Kutta scheme, and the mb, Tb, Tu, and burning rate, dmb/dt, can beobtained. During the combustion process, gas compositions andproperties are calculated through chemical equilibrium with 11species and 7 equations [19].

Fig. 5. Combustion pressure and normalized mass burning rate versus the time atthree initial temperatures.

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The normalized mass burning rate (NMBR) is defined as 1m �

dmbdt ,

where m is the total mass of combustible mixture and mb refersto the mass of burned gas. The NMBR rate is calculated from thetwo-zone model described above. This parameter reflects the burn-ing velocity of the mixture during the combustion process.

The flame development duration is defined as the time intervalfrom the spark ignition to the timing of 10% accumulated massburned [20]. This parameter can be used to reflect the flame earlydevelopment. The combustion duration is defined as the timeinterval from the spark ignition to the timing of 90% accumulatedmass burned.

The study covers the equivalence ratios ranging from 0.7 to 1.8,nitrogen dilution ratios of 0.05, 0.1 and 0.15, initial temperatures of373, 423 and 473 K, and initial pressures of 0.1, 0.25, 0.5 and0.75 MPa.

3. Results and discussions

Fig. 4 gives the combustion pressure and the normalized massburning rate versus the time for the methanol-air mixtures at differ-ent equivalence ratios. At the initial temperature of 423 K and theinitial pressure of 0.1 MPa, the fastest normalized mass burning rateis presented at the equivalence ratio between 1.0 and 1.2 along withthe shortest time interval from the ignition start to the peak pressureor peak mass burning rate. This reveals the fact that the fastest flame

Fig. 6. Combustion pressure and normalized mass burning rate versus the time atfour initial pressures.

speed is presented at this equivalence ratio. In the case of rich mix-ture combustion, the flame propagation speed decreases and thecombustion duration increases due to insufficiency of oxygen. Forlean mixture combustion, the decrease in flame propagation speedand amount of fuel result in the decrease of peak pressure and theslowing of normalized mass burning rate. The phenomenon be-comes more obviously for further richer or leaner mixtures.

Fig. 5 shows the combustion pressure and normalized mass burn-ing rate at three initial temperatures for the stoichiometric metha-nol-air mixtures. The results show that the normalized massburning rate increases with the increase of the initial temperature.The time interval from the start of ignition to peak pressure de-creases with the increase of the initial temperature. The increase ofthe flame propagation speed with the increase of the initial temper-ature is responsible for this. The peak pressure decreases with the in-crease of the initial temperature, and this is resulted from thedecrease of fuel density and the amount of heat released with the in-crease of the initial temperature when the initial pressure is fixed.

Fig. 6 illustrates the combustion pressure and the normalizedmass burning rate for the stoichiometric methanol-air mixture atfour initial pressures. The peak combustion pressure and the timeinterval from the start of ignition to peak pressure increase withthe increase of the initial pressure, while the normalized mass burn-ing rate decreases with the increase of the initial pressure. Theamount of fuel will be increased with the increase of the initial

Fig. 7. Combustion pressure and normalized mass burning rate versus the time atdifferent dilution ratios.

Z. Zhang et al. / Applied Thermal Engineering 29 (2009) 2680–2688 2685

pressure leading to the increase of the peak pressure. Meanwhile,increasing the initial pressure decreases the flame propagationspeed, thus the normalized mass burning rate is decreased and thetime interval from the start of ignition to peak pressure is increased.

Fig. 7 gives the combustion pressure and normalized massburning rate for the stoichiometric methanol–air mixture underdifferent dilution ratios at the initial temperature of 373 K andthe initial pressure of 0.1 MPa. The results show that the peak com-bustion pressure and the normalized mass burning rate decreasewith the increase of dilution ratio whereas the time interval fromthe ignition start to peak pressure increases with the increase ofthe dilution ratio. Increase in dilution ratio will decrease theamount of fuel and flame propagation speed, leading to the de-crease of normalized mass burning rate and the increase of timeinterval from ignition start to peak pressure.

Fig. 8 illustrates the effects of initial temperature on the flamedevelopment duration, the combustion duration, the maximumcombustion pressure and temperature for the methanol–air mix-tures at the initial pressure of 0.1 MPa. The shortest flame develop-ment duration and combustion duration are presented at the sameequivalence ratio of 1.1. For lean mixture combustion, both theflame development duration and the combustion duration increasewith the decrease of the equivalence ratio, and for rich mixturecombustion, the flame development duration and the combustionduration increase with the increase of the equivalence ratio. The

Fig. 8. Combustion parameters versus the equiv

increase of flame development duration and combustion durationat both lean mixtures and rich mixtures are due to the decreasedflame speed for a given equivalence ratio, the flame developmentduration and the combustion duration decrease with the increaseof initial temperature. This reflects the fact that high initial tem-perature will increase the flame propagation speed, leading toshort flame development duration and combustion duration. Forthe fixed initial pressure, increasing the initial temperature will de-crease the peak combustion pressure, and this is due to the de-crease of fuel density and amount of heat released. The studyalso reveals that maximum combustion temperature showsslightly increase with the increase of the initial temperature, andthis suggests that maximum combustion temperature is insensi-tive to the variation of initial temperature. The increase of flamepropagation speed and decrease of temperature are contributedto this behavior. The results show that the shortest flame develop-ment duration and combustion duration are decreased by 32% and30%, respectively, as the initial temperature increases from 373 to473 K. The maximum value of peak combustion pressure is de-creased by 15%, while the maximum value of peak combustiontemperature is increased by 0.7% as the initial temperature in-creases from 373 to 473 K.

Fig. 9 shows the flame development duration, the combustionduration, the peak combustion pressure and temperature of themethanol–air mixtures at three initial pressures. The results show

alence ratios at three initial temperatures.

2686 Z. Zhang et al. / Applied Thermal Engineering 29 (2009) 2680–2688

that the flame development duration and the combustion durationgive the shorter period at low initial pressure. In the case of leanmixture combustion, the flame propagation speed decreases withthe increase of initial pressure, thus, the flame development dura-tion and combustion duration show an increase with the increaseof initial pressure. For rich mixture combustion, the flame tends tothe diffusional-thermal instability, and the hydrodynamic instabil-ity is enhanced with the increase of initial pressure due to the de-crease of flame thickness. Cellular flame structure is easy to occurat the elevated initial pressure and rich mixture, leading to the in-crease of flame speed as increasing the flame front area betweenthe unburned and the burned gases. Thus, the flame developmentduration and combustion duration increase with the increase ofinitial pressure (from 0.1 to 0.25 MPa) and they decrease with fur-ther increasing the initial pressure (from 0.25 to 0.5 MPa).

The peak combustion pressure remarkably and the peak tem-perature increase slightly, this is due to the increase in amountof fuel as initial pressure increases. The peak combustion pressureand temperature are presented at the equivalence ratio of 1.1regardless of the initial pressures. Increasing the initial pressure di-rectly increases the peak pressure, but increasing the initial pres-sure leads to equivalent increase in the amount of fuel and theamount of air, resulting in the small influence on peaktemperature.

Fig. 9. Combustion parameters versus the equ

Fig. 10 gives the flame development duration, the combustionduration, the peak combustion pressure and temperature for meth-anol–air–diluent mixtures at different dilution ratios. The flamedevelopment duration and the combustion duration increase withthe increase of the dilution ratio, reflecting the decrease of theflame propagation speed as the diluent is added. The peak combus-tion pressure and temperature decrease with the increase of thedilution ratio, and this is due to the decrease of fuel amount andheat released. Moreover, the diluent will absorb part of the re-leased heat and lower the temperature. The combustion durationgives its minimum value and the peak pressure and temperaturegive their maximum values at the equivalence ratio of 1.1 regard-less of the dilution ratio. Flame development duration and com-bustion duration are increased with the increase of dilution ratio.At ur = 15%, the shortest flame development duration is double tothat of the mixture without the diluent gas. The maximum valuesof peak combustion pressure and temperature at ur = 15% are de-creased by 14% and 0.6%, respectively, comparing with those with-out diluent gas addition.

4. Conclusions

Combustion characteristics of methanol–air and methanol–air–diluent mixtures at different equivalence ratios, initial pressures

ivalence ratios at three initial pressures.

Fig. 10. Combustion parameters versus the equivalence ratios at different dilution ratios.

Z. Zhang et al. / Applied Thermal Engineering 29 (2009) 2680–2688 2687

and temperatures, and dilution ratios are studied. The main con-clusions are summarized as follows:

(1) The methanol–air mixtures slightly richer than stoichiome-tric equivalence ratio give the peak combustion pressure,peak mass burning rate and burned gas temperature givetheir maximum values at the equivalence ratio of 1.1, whilethe shortest flame development duration and combustionduration are presented at the equivalence ratio of 1.1.

(2) Flame development duration and combustion durationdecrease with the increase of initial temperature. The shortestflame development duration and combustion duration aredecreased by 32% and 30%, respectively, as the initial temper-ature increases from 373 to 473 K. Peak combustion pressuredecreases with the increase of initial temperature, while themaximum burned gas temperature increases with increasingthe initial temperature. The maximum value of peak combus-tion pressure is decreased by 15%, while the maximum valueof peak combustion temperature is increased by 0.7% as theinitial temperature increases from 373 to 473 K.

(3) Flame development duration and combustion duration areincreased with the increase of dilution ratio. At ur = 15%,the shortest flame development duration is double to thatof the mixture without the diluent gas. Peak combustion

pressure and temperature are decreased with the increaseof dilution ratio, and the maximum values of peak combus-tion pressure and temperature at ur = 15% are decreased by14% and 0.6%, respectively, comparing with those withoutdiluent gas addition.

Acknowledgments

This study was supported by National Nature Science Founda-tion of China (50521604, 50876086).

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